To be suitable for device fabrication, layers of semiconductor materials must be of high purity and have low densities of defects or dislocations. Preparation of quality layers of certain materials, especially compound materials, has been hindered in the past because of a lack of quality bulk single crystals and also due to inadequate substrates for hetero-epitaxial growth.
Such problems have hindered the development of gallium nitride (GaN); of other Group III nitrides, including, e.g., AlN, InN, GaInN, and other mixed nitrides (referred to herein as “III nitrides”); of certain Group III-V compounds; and of certain other compound materials (e.g., II-VI materials) generally. For example, the III nitrides have semiconductor properties that are advantageous for fabrication of electronic components (e.g., high temperature FETs), optic components (e.g., short wavelength LEDs and lasers), and mixed opto-electronic components (e.g. photovoltaic devices). However, the preparation of quality layers of the aforementioned materials has been hindered by a lack of quality bulk crystals and/or suitable substrates that match the crystal properties of these materials. A substrate that does not closely match the crystal properties of the materials to be grown thereon can lead to an unacceptable density of defects and dislocations (for GaN, particularly threading dislocations (TD) originating at the interface between the substrate and the GaN).
In the case of GaN, it should be noted that crystal quality can be improved by substrate pre-treatment such as chemical modification of the substrate surface, e.g., by nitrodization; growing, often at lower temperatures (LT), a thin buffer layer of, e.g., AlN or GaN, thermal annealing, and the like. Crystal quality has also been improved by exploiting epitaxial lateral overgrowth (ELO), during which a layer grows laterally over a masking layer as well as vertically from the substrate. See, e.g., U.S. Pat. No. 6,153,010. Known ELO processes grow GaN on substrates that are partially covered by a photo-lithographically patterned mask and under conditions promoting, first, growth on the regions of the substrate exposed through the mask opening, and second, growth laterally over the mask
Also, such problems have hindered the development of alloys of silicon (Si) and germanium (Ge). Such bulk crystals are generally not available. However, improved crystal quality has been obtained by growing these materials on buffer layers having a composition that grades from that of the substrate (e.g., Si or Ge) to that of the material to be grown (e.g., SiGe).
In addition to defect or dislocation density, another important crystal property, especially for the III nitrides (e.g., GaN), is “crystal polarity”, See, e.g., Sumiya et al., 2004, Review of polarity determination and control of GaN, MRS Internet J. Nitride Semicond. Res. 9, 1. GaN crystal polarity is illustrated in FIG. 1 (where Ga atoms are illustrated by large grey spheres, N atoms are illustrated by small black spheres, and bonds are illustrated by double lines). As illustrated, in wurtzite GaN (and the other III nitrides) each Ga atom is tetrahedrally coordinated to four nitrogen (N) atoms, but of these, it is strongly bonded only to the three nearest-neighbor Ns (indicated in FIG. 1 by “*”). If the three strong bonds from a Ga to its three nearest-neighbor Ns are directed downward towards the substrate, then the polarity is +c (also known as Ga-face), where the label c refers to the crystal plane perpendicular to the plane of the epitaxial film. For the opposite polarity −c (also referred to N-face), the direction of the Ga to its three nearest-neighbor Ns is directed upwards towards the growth direction.
It is important to note that the polarity of the material is not a surface property and seriously affects the bulk properties of the GaN (or other nitride material) and it is often advantageous to fabricate components in one or the other polarity. Therefore it is often desirable to select the polarity of the epitaxial growth layer to tailor it to a specific application, for example layers with +c polarity are often preferred for III nitride component fabrication.
A further important crystal parameter, especially for hetero-epitaxial growth of III-V nitride films, is the induced strain in the epitaxial layer resulting from the lattice mismatch between a non-native substrate and the nitride layer (e.g. ≈15% between Sapphire and GaN). The strain induced in the nitride epitaxial layer can manifest itself physically in a number of significant ways, including but not limited to defect/dislocation formation, compositional phase separation and internal polarization field creation.
Phase separation and piezoelectric field creation have a detrimental effect on light emitting devices fabricated from the III nitride materials system, in particular for active layers grown from the InGaN material. The components of the binary compound InGaN, namely InN and the GaN are not fully miscible and therefore under a given set of growth conditions and film thickness there exists a fixed range of energetically favorable InGaN compositions. The introduction of lattice strain and defects into the InGaN system can result in thicker InGaN layers grown at energetically unfavorable compositions tending to phase separate i.e. the In and Ga atoms will not be homogenously distributed throughout the layer. The non-homogeneity can result in a distributed perturbation of the band-gap energy of the material i.e. the phase separated regions can act disproportionately as optical absorption centers or optical scattering sources, which can result in a deterioration of the internal quantum efficiency (IQE) of the nitride device. The IQE, the number of photons generated within the active layer divided by the number of electrons pumped into the device, has been observed to decrease rapidly with increasing indium component in the InGaN active region and this phenomena has been related to the phase separation of the material. The decrease in IQE has considerable implications for applications where a large indium component would be required, for example, long wavelength emitters fabricated from III nitride materials.
The combination of the wurtzite crystal structure of the III nitrides and the common growth orientation in the (0001) plane produces polarization fields in the material. The polarization fields subsequently produce electrostatic charge densities in the material that have an influence on carrier distribution and electric fields in nitride devices. The polarization fields in the nitrides have been shown to originate from two separate components, composition related (spontaneous) and strain related (piezoelectric). The induced charges cause interface energy barriers that can impede the transport of electron and holes into the active region of a device. The spontaneous and piezoelectric polarizations separate the electron and hole spatial distribution due to an incline of the band structure, this separation of carriers results in a significant reduction in the efficiency of the device.